Building materials and structures made of iron or steel, including towers and bridges, are coated with molten zinc for protection against corrosion. The material to be coated is subjected to pretreatment by a process comprising the steps of degreasing, rinsing with water, pickling, rinsing with water, and flux treatment. The flux treatment of the steel to be coated is usually carried out by a dry process comprising dipping it in an aqueous flux solution, and drying it, though there is also a wet process.
It has hitherto been usual to employ as the flux an aqueous solution containing zinc chloride and ammonium chloride, or only ammonium chloride, as disclosed in Japanese Patent Application Laid-Open No. 136759/1983. Ammonium chloride is decomposed into hydrogen chloride and ammonia at a galvanizing temperature, and the ammonia combines with zinc chloride to form zinc monoaminechloride. The hydrogen chloride and zinc monoaminechloride are so corrosive as to remove from the surface of the steel to be coated the rust which has formed on the steel surface after its flux treatment, and the zinc oxide which the steel to be coated has drawn in from the surface of a zincbath when it has been dipped in it. Thus, they contribute to improving the wetting of the cleansed steel surface with molten zinc to enable the formation of a good zinc coating on the steel surface.
The recent development of an oceanic, or coastal region has given rise to the necessity for the construction or installation of road facilities, line hardware, and building structures in a highly corrosive environment. Various attempts have, therefore, been made to protect those facilities or structures by coating them with a galvanizing layer, or a thick layer of a paint, but none of them has been satisfactory. The problem of acid rain has added to the necessity for the effective rustproofing of steel structures. A snowy region has another problem. A snow melting agent, such as calcium chloride, causes the corrosion of steel. There are strict standards calling for the protection of steel sheets for automobiles against corrosion by a snow melting agent, as represented by a Canadian court decision.
A great deal of research work has, therefore, been made to obtain a coating giving a higher degree of corrosion resistance by electroplating or molten metal coating. A molten alloy of zinc and aluminum has, among others, drawn attention as being able to form a coating providing a high degree of corrosion resistance, and is already used to some extent or other for coating steel sheets. It is employed in a continuous coating process which is carried out in a non-oxidizing atmosphere.
There is also known a two-stage coating process which is employed for coating small parts, such as steel wire and line hardware, in an atmospheric environment. This process comprises coating the material with molten zinc, and coating it again immediately with a molten alloy of zinc and aluminum to form a coating of the alloy. The process, however, has a number of drawbacks including the necessity for the installation of a bath of the molten alloy of zinc and aluminum, the resulting increase in the space for installation and the cost of maintenance, and the operating time prolonged by the repeated coating operation.
There is hardly known any case where a single-stage coating process employing a molten alloy of zinc and aluminum has been carried out in an atmospheric environment, though hot dip galvanizing has always been done in such an environment. This means that it has been impossible to obtain a satisfactory coating of an alloy of zinc and aluminum even by using the flux as disclosed in Japanese Patent Application Laid-Open No. 136759/1983, if the process is carried out in an atmospheric environment.
The inability of any single-stage process to form a satisfactory coating from a molten alloy of zinc and aluminum in an atmospheric environment is due to the selective oxidation of aluminum which occurs in the surface of the zinc bath and prevents any satisfactory contact between the steel to be coated and the constituents of the bath, and also to the fact that the zinc chloride and ammonium chloride which are used as the flux in ordinary hot dip galvanizing coating undergo the following reactions: EQU 3ZnCl.sub.2 +2Al.fwdarw.3Zn+2AlCl.sub.3 EQU 6NH.sub.4 Cl+2Al.fwdarw.2AlCl.sub.3 +6NH.sub.4 +3H.sub.2
These reactions lower the effect of the flux and result in an unsatisfactory coating having bare spots, roughness, or lumpiness.
Although it has been known that the use of a zinc both containing aluminum at a high concentration yields a coating of improved corrosion resistance, the absence of any appropriate flux has made it difficult to form any satisfactory coating on an industrial basis.